// CHEMISTRY L5 — THERMODYNAMICS

Some reactions run on their own; others never will. The deciding factor is Gibbs free energy — a tug-of-war between energy and disorder that temperature can tip either way. Turn the dial and find the tipping point.

// FIG. 01 — THE GIBBS FREE-ENERGY CALCULATOR

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// ENTROPY — THE ARROW OF DISORDER

Entropy (S) measures how spread-out and disordered energy and matter are. The universe relentlessly tends toward higher entropy — gases fill their container, hot things cool, sugar dissolves. A reaction that makes more gas molecules, or turns a solid into a liquid or gas, has a positive ΔS (more disorder). The second law of thermodynamics says the total entropy of the universe always increases in any real process.

// GIBBS FREE ENERGY — THE VERDICT

Gibbs combined both ideas into one number: ΔG = ΔH − TΔS. A reaction is feasible (spontaneous) when ΔG is negative. Notice the tug-of-war: a negative ΔH (exothermic) helps, and a positive ΔS (more disorder) helps — but the entropy term is multiplied by temperature. So raising the temperature can flip a reaction from impossible to spontaneous (or vice versa), and the crossover temperature where ΔG = 0 is exactly T = ΔH ÷ ΔS. That's why limestone only decomposes in a hot kiln, and why ice melts above 0 °C but not below.

// REFERENCES & FURTHER READING

Wikipedia — Gibbs free energy ↗ Wikipedia — Entropy ↗ BBC Bitesize — Science (chemistry) ↗

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